Read the Character Creation rules (link) if you haven't already. Read them again if you've already read them. If you read them now and still haven't come up with an answer to your question, read them a third time.
Specifically section 2.1, "Select Handle", if the link wasn't clear enough.
I recently submitted a screen name a that was rejected and am curious as to why. Kieran is a real first name and Horne is a real last name... so why was the name Kieran Horne rejected? if it is already taken, fine, tell me and I'll choose a new name. Thank you for your time,
It would help if you listed the exact reason given in the e-mail. However, if I were to guess, I would think that the person declining it was thinking of the canon surname Horn, which is disallowed. It's at the discretion of the team member if they think a name is sufficiently close to a canon name as to be attempting to circumvent the rules.
"May the Grace of Ara go with you, and His Vengeance be wrought upon your enemies."